The present invention relates to the art of induction heating and more particularly to a unique compact induction heating system for use under the hood or cowling of internal combustion engine drive implement.
Induction heating involves the use of an induction heating coil that is driven by alternating currents to induce voltage and thus current flow in a work piece encircled by or associated with the induction heating coil. Such technology has distinct advantages over convection heating, radiant heating and conduction heating in that it does not require physical contact with the heated work piece or circulating gasses to convey combustion type heat energy to the work piece. Consequently, induction heating is clean, highly efficient and usable in diverse environments. However, induction heating by work piece associated conductors normally involve power supplies connected to an AC line current. Such heating power supplies are constrained by the frequency of the incoming line. In some instances, the line voltage is three phase, which is rectified to produce a DC link and then converted to alternating current by use of an inverter.
Such DC link driven power supplies have two distinct disadvantages. They are relatively large and involve a heavy core that constitutes a major component of the input rectifier. Consequently, such power supplies cannot be fit into a small compartment, such as the area under the hood of a motor vehicle. Further, a heating system to be used in association with an internal combustion engine cannot involve induction heating since there is no source of alternating current to drive the power supply for the induction heating coil.
The present invention overcomes the disadvantages associated with existing induction heating systems, wherein the system can be made quite compact so that it is capable of being located in a small compartment, such as the under hood of a motor vehicle or other internal combustion engine driven implements.
The present invention utilizes a compact inverter having a clean DC input and components which fit into a relatively small housing with a volume of less than about 100 cubic inches. By developing a special induction heating system for use in a confined space, the advantages of induction heating can be employed for various heating functions, in such confined space as under the hood of a motor vehicle. Consequently, the required heating operations in such a confined space can enjoy the advantages of induction heating with its efficiency, environmental friendly nature, and ease of control.
In accordance with the present invention, there is provided a compact induction heating system for use on an internal combustion engine driven implement having an engine driven alternator to generate DC current for storage in a battery used as a source of clean DC current of less than 50 volts for ignition of fuel in the engine. The system comprises a high frequency inverter with an input connected to the clean DC source. A pair of identical AC tuning capacitors are connected in series across the clean DC source. Each capacitor is initially charged to one half the input DC voltage. The load inductor is connected at one end to the center junction of the two AC capacitors. A pair of solid state switches (i.e. IGBT transistors) are also connected in series across the clean DC source and in parallel with the two series AC capacitors. The other end of the inductor is connected to the center junction of the two switches. The switches are opened and closed (gated on and off) alternately at a frequency determined by the application (typically between 10 kHz and 20 kHz, but with a range capability of 1 kHz to 200 kHz). The frequency of the gates is equal to the natural resonant frequency of the load. The power or the amount of heat generated can be varied by slightly adjusting the gating frequency above or below the natural resonant frequency of the load. When the first switch closes, the voltage stored in the first AC capacitor is discharged through the inductor, producing one half of the AC sinusoidal current, and back to the opposite polarity of the clean DC source. At the same time, the first capacitor is then charged to the full potential of the clean DC source. The switch is then opened (turned off), and after a sufficient amount of dead time has elapsed, the second switch is turned on. When the second switch is closed, the second AC capacitor then discharges through the inductor, producing the other half of the AC sinusoidal current, and is then charged to the full potential of the clean DC source, but in the opposite polarity of the other capacitor. This process is then repeated as long as the gate signals are present. The subsequent cycles after the first cycle differ in the fact that the AC tuning capacitors are now charged to the full potential of the clean DC input. The process is halted when the gating signals are removed or disabled. The AC current generated by the capacitor-transistor switching system (inverter) is passed though the inductor. This current induces a voltage within the part/workpiece to be heated (via magnetic flux). The induced voltage develops a current within the part which meets resistance to the material which comprises the part. This resistance to current flow generates heat in the form of I2R losses, where (I) is the induced current and (R) is the resistance of the part. The heat developed in the part can be measured in watts (W). W=I2R. The load inductor is preferably the actual induction heating coil whereby the natural frequency of the two current paths is equal to the driven frequency of the switching circuit. As an alternative, the single inductor is the primary of an output transformer so that the heat controlling driven frequency can be delivered to inductors that are smaller or larger than the nominal inductor. In accordance with another aspect of the present invention the DC current source is the alternator of the engine when the engine is driven and the battery of the engine when the internal combustion engine is not operating.
In accordance with still a further aspect of the present invention the clean DC voltage is preferably in the range of 12 to 24 volts DC which is substantially less than 20 volts and the general upper limit of 50 volts DC. The power supply has a lower input limit of 6 volts DC. In one aspect of the invention, the inductor of the inverter is an induction heating coil. In an other aspect, the inductor is a primary winding of an output transformer having a secondary winding forming the induction heating coil. Although the frequency of the heating system can be as low as 1.0 kHz, it is preferably in the range of 10-20 kHz to drastically reduce this size of those components constituting the inverter. By such high frequency control of the gating circuit, the housing for the inverter can be reduced to substantially less than 100 cubic inches so that it easily fits under the hood of a motor vehicle or the cowling an internal combustion driven implement. The heating system is preferably driven by a switching circuit operated between 10 kHz and 20 kHz. By this high frequency operation, the compactness of the inverter is possible. The advantage of an induction heating system of the type to which the present invention is directed is the ability to operate at a high frequency to produce a relatively low reference depth of heating by the output induction heating coil for efficient heating of related constituents within a very confined compartment.
In accordance with another aspect of the present invention, the gating circuit has a two state counter with an adjustable oscillator for adjusting the driven frequency to tune the actual output heating of the system. In this gating circuit, there are alternate gating pulses with an adjustable dead band between the pulses to operate the first and second switches.
In accordance with another aspect of the present invention, there is a dead time between the pulses to allow the natural frequency of the two combined conductive paths to prepare for reversing of the switches. This is another advantage of using high frequency. The dead time can be reduced between the pulses that control the driven frequency determining the actual heating output of the novel induction heating system.
The primary object of the present invention is the provision of a compact induction heating system that can be mounted in a confined area for diverse operations of induction heating in such confined areas.
Yet another object of the present invention is the provision of a compact induction heating system, as defined above, which compact induction heating system is operated at a high frequency so that it can be mounted in a relatively small housing, such as a housing having a volume of less than about 100 cubic inches.
Still a further object of the present invention is the provision of a compact induction heating system, as defined above, which system utilizes a unique high frequency operated inverter for converting clean DC current to the high frequency heating current. A clean DC current is a current that is not generated by a rectifier and thus has a minimal ripple factor that will adversely effect the operation of the high frequency inverter. Such clean DC is available in an implement or vehicle driven by an internal combustion engine wherein the DC current is generated by an alternator and stored in a battery for use in the emission system of the internal combustion engine.
These and other objects and advantages will become apparent from the following description of the present invention utilizing the accompanying drawings.